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Rev counter reading half!

Discussion in 'Electronic Basics' started by Tim2cv6, May 25, 2004.

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  1. Tim2cv6

    Tim2cv6 Guest

    I have recently bought a rev counter for an old car I am currently
    restoring. The electronic problem I am having is the rev counter seems
    to be reading about half of what it should be! Is there an easy fix to
    this by means of changing resistors or am I stuck! I have taken the
    back off it and can see a large resistor coloured brown with green,
    black, pinky/orange? then silver banding. this is directly attached to
    the red +ve input wire. I have pictures if needed to makes more sense.
    Any ideas greatfully recieved!
  2. If that resistor is in series with a meter movement (not the
    electronic circuit between the meter and the ignition system) then you
    may be able to change it and recalibrate the meter. The resistor you
    describe appears ot be a 15k ohm resistor (brown=1, green=5, orange=3,
    so 15 followed by 3 zeros, silver=10% tolerance)

    Before you remove this one, you might do an experiment. Parallel it
    with a 47k resistor (yellow, purple, orange) and see if you get the
    expected 11% or so increase in the meter reading. This experiment is
    not likely to hurt anything if his is not the purpose of the resistor,
    since it makes a total resistance change not much more than the
    tolerance of the 15k resistor allows. If this works, then paralleling
    the 15k with another 15k should almost double the reading.
  3. JeB

    JeB Guest

    I'd begin by looking closer for something to select between 4, 6 and 8
    cyl. engines.
  4. Rob Paisley

    Rob Paisley Guest

    How many cylinders does your car have?

    Does the tachometer have an adjustment for the number of cylinders?

  5. Tim2cv6

    Tim2cv6 Guest

    ok, thanks, another chain of thought would be to look at the signal
    going to the rev counter from the electronic ignition. Is there any
    way to convert the signal from 1 pulse to 2 pulses hence changing the
    value of the signal to twice what it is at the moment. Thanks
  6. ~^Johnny^~

    ~^Johnny^~ Guest


    Most simple Tachs are just freq counters, using a double diode-cap network.
    It uses differentiating and integrating functions on the input pulses, to
    yield an average voltage, which is read on the D'Arsonval meter. D'Arsonval
    movements actually respond to current... the problem is: these diode-cap counter type tachs assume twice the
    frequency, due to the differentiating action. On an old style "kettering"
    ignition system, this works fine, as the tack will double the frequency of
    the incoming pulses off the points.. But with electronic ignition, the
    waveform seen by the tach is radically different. The tach may no longer be
    frequency doubling, so the average voltage seen by the
    meter-movement-series-resistor combination might be halved. Either the
    leading or trailing edge of the pulse train isn't abrupt enough, so the tach
    sees only "half frequency".

    It looks like the series resistor is 51 Kohms. So try another 51 Kohm
    resistor in parallel with it. If little change, then it's 510 ohms, so use
    a 510 ohm resistor across it instead.

    Check the linearity when you're done. Simultaneously hook up a tach with
    known accuracy (borrow one), and check it at three or four different speeds,
    like curb idle, cruising, power band, and a little bit below redline.
    wide-open at throttle dot info

    "The first step in intelligent tinkering is to
    save all the parts." - Aldo Leopold
  7. ~^Johnny^~

    ~^Johnny^~ Guest


    Also, the resistor could be 50K, 56K, 500 Ohm, or 560 Ohm.

    Sometimes blue (6) can look black (0) or gray (8) if you are slightly color
    blind, and don't know it, or if you are viewing it in poor lighting.
    Fluorescent is generally the worst.

    Here's what I want you to do:

    Cut one of the leads of that resistor, right in the middle, and then measure
    its resistance. You can later tack it back together easily enough with a bead
    of solder. Then find another resistor / potentiometer to parallel it, to cut
    its value in half.

    wide-open at throttle dot info

    Always listen to experts. They will explain what
    can't be done and why. Then do it. - Robert Heinlein
  8. ---------------
    Send the signal to ground through a resistor, inline through a cap
    off the top of that resistor to a second resistor to ground, then
    full-wave rectify the resultant pseudo-AC voltage with an op-amp
    fullwave rectifer that clips the bottom off with and use a Schmitt
    trigger on the remaining pulses.

  9. ~^Johnny^~

    ~^Johnny^~ Guest

    Why not just build a new &^##%@* tach from scratch? ;->
    wide-open at throttle dot info

    "The first step in intelligent tinkering is to
    save all the parts." - Aldo Leopold
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